I tried out the Asus VivoTab RT at Best Buy over the weekend. It is a very slick device. Great display, very responsive performance. The 10" screen is a bit small for me, but my wife said she loved it. The keyboard dock also has a battery inside, so it gives a total 13 hours of battery life when docked. She's not worried about backward compatibility, so Windows RT would be great for her.
For anyone interested in the U.S., OfficeMax is selling the VivoTab RT for $50 off this week. Asus is also giving out free keyboard docks ($160 value) through the end of December on their website.
I also tried the Lenovo Yoga. While the hinges are neat, I'm not too sure about having the keyboard exposed. I can picture the keys getting damaged when someone slides it across a table in tablet mode. Plus, at the $1000 level, I'm more inclined to wait for the various x86 tablets to come out.